Allison T56 longest-lived engine design?
The same basic Allison T56 engine (currently a Rolls-Royce engine, of course) that powered the 1954 C-130 prototype still powers the C-130J.
Can anybody suggest to me any other aircraft powerplant that has been in production and productive, mainstream use for as long as the T56? (I'm writing about the C-130 for a U.S. aviation magazine, so I need somebody to stop me from making an outrageous longevity claim before learning that there's a Russian radial that has been in production since 1926...)